1. Field of the Invention
This invention relates to an improved method for establishing and maintaining electric continuity through a drill string using an insulated electric conductor. In one aspect, it relates to a method and apparatus for preventing the cable stored in a rotary drill string from twisting or snarling attendant to drilling operations.
2. Description of the Prior Art
In the drilling of oil wells, gas wells, and similar boreholes, it frequently is desirable to transmit electric energy between subsurface and surface locations. One application where electrical transmission has received considerable attention in recent years is in wellbore telemetry systems designed to sense, transmit, and receive information indicative of a subsurface condition. This operation has become known in the art as "logging while drilling."
A major problem associated with wellbore telemetry systems has been that of providing reliable means for transmitting an electric signal between the subsurface and surface locations. This problem can best be appreciated by considering the manner in which rotary drilling operations are normally performed. In conventional rotary drilling, a borehole is advanced by rotating a drilling string provided with a drill bit at its lower end. Lengths of drill pipe, usually about 30 feet long, are added to the drill string, one at a time, as the borehole is advanced in increments. In adapting an electric telemetry system to rotary drilling equipment, the means for transmitting the electric signal through the drill string must be such to permit the connection of additional pipe lengths to the drill string as the borehole is advanced.
An early approach to the problem involved the use of continuous electric cable which was adapted to be lowered inside the drill string and to make contact with a subsurface terminal. This technique, however, required withdrawing the cable from the drill string each time a pipe length was added to the drill string. Another approach involves the use of special drill pipe. Each pipe section of the special pipe is provided with an electric conductor having connectors at its opposite ends. Electric continuity is maintained across the junction of two pipe sections by connectors of one section contacting a connector on the adjacent pipe section (see U.S. Pat. Nos. 3,518,608 and 3,518,609). Disadvantages of this system include the high cost of the special pipe sections, the need for a large number of electric connections (one at each joint), and the difficulty of maintaining insulation of the electric connectors at each joint.
Still another approach involves the use of cable sections mounted in each pipe section (See U.S. Pat. No. 2,748,358). The cable sections are connected together as pipe sections are added to the drill string. Each cable section is normally made slightly longer than its associated pipe section, with the result that a small amount of slack is present in the conductor string at all times. Drilling fluid flowing through the drill string exerts a fluid drag on the loose cable which tends to damage the connectors or snarl the cable.
A more recent development in cable systems for wellbore telemetry operations is described in U.S. Pat. No. 3,825,078 on "Method of Mounting and Maintaining an Electric Conductor in a Drill String." The cable system disclosed in this patent employs a looped cable which permits the cable string to be extended as the drill string is lengthened. Experience with this system has indicated that the overlapped cable sometimes becomes entangled as a result of pipe rotation or fluid flow in the pipe string.
A method for preventing looped cable from tangling is described in U.S. Pat. No. 3,825,079 on "Method For Mounting An Electric Conductor In A Drill String". In accordance with one embodiment of this patent, entanglement of the overlapped cable is minimized by providing a track between a fixed upper support and lower axially movable weight. The support and weight maintain the cable in an overlapped configuration, allowing stored cable to be withdrawn as drilling progresses, but minimizes twisting of the looped cable. However, this system is cumbersome since it requires long track sections which must be lowered into and withdrawn from the pipe string. Furthermore, this somewhat limits the length of cable which can be stored.